U.S. patent application number 11/568479 was filed with the patent office on 2008-10-02 for optical reader/writer with dedicated focus tracking beam.
This patent application is currently assigned to KONINKLIJKE PHILIPS ELECTRONICS, N.V.. Invention is credited to Christopher Busch, Alexander Marc Van Der Lee.
Application Number | 20080239897 11/568479 |
Document ID | / |
Family ID | 34965721 |
Filed Date | 2008-10-02 |
United States Patent
Application |
20080239897 |
Kind Code |
A1 |
Busch; Christopher ; et
al. |
October 2, 2008 |
Optical Reader/Writer With Dedicated Focus Tracking Beam
Abstract
The present invention relates to an optical reader/writer for a
two dimensional storage disc, comprising means (21, 22, 23, 14, 16,
15) for generating a plurality of laser beams and projecting said
beams onto a rotating disc, means (24, 25, 26) for detecting the
beams after being diffracted by the disc, and means (24, 27, 28)
for determining a focus error signal (29) based on one of said
plurality of beams. The plurality of beams comprises an array of
beams having a first wavelength, and a dedicated focus tracking
beam having a second wavelength, and the focus error signal (29) is
based on said focus tracking beam. According to the invention,
beams having one wavelength is used for the actual accessing of the
data on the disc, while a beam having a second wavelength is used
for focus tracking. The focus tracking can then be based on one
single beam without interference from adjacent beams.
Inventors: |
Busch; Christopher;
(Eindhoven, NL) ; Van Der Lee; Alexander Marc;
(Eindhoven, NL) |
Correspondence
Address: |
PHILIPS INTELLECTUAL PROPERTY & STANDARDS
P.O. BOX 3001
BRIARCLIFF MANOR
NY
10510
US
|
Assignee: |
KONINKLIJKE PHILIPS ELECTRONICS,
N.V.
EINDHOVEN
NL
|
Family ID: |
34965721 |
Appl. No.: |
11/568479 |
Filed: |
April 27, 2005 |
PCT Filed: |
April 27, 2005 |
PCT NO: |
PCT/IB05/51374 |
371 Date: |
October 30, 2006 |
Current U.S.
Class: |
369/47.49 ;
G9B/7.067; G9B/7.113 |
Current CPC
Class: |
G11B 7/0903 20130101;
G11B 2007/0013 20130101; G11B 7/1275 20130101; G11B 7/1353
20130101 |
Class at
Publication: |
369/47.49 |
International
Class: |
G11B 7/12 20060101
G11B007/12 |
Foreign Application Data
Date |
Code |
Application Number |
May 4, 2004 |
EP |
04101910.0 |
Claims
1. An optical reader/writer for a two dimensional storage disc,
comprising: means (21, 22, 23, 14, 16, 15; 21, 22, 12, 30) for
generating a plurality of laser beams and projecting said beams
onto a rotating disc, means (24, 25, 26; 31, 32, 25, 26) for
detecting the beams after being diffracted by the disc, and means
(24, 27, 28; 31, 33, 27, 28) for determining a focus error signal
(29) based on, one of said plurality of beams, characterized in
that said plurality of beams comprises an array of beams having a
first wavelength, and a dedicated focus tracking beam having a
second wavelength, and wherein said focus error signal (29) is
based on said focus tracking beam.
2. An optical reader/writer according to claim 1, wherein said
focus tracking beam coincides with one of the beams, preferably the
central beam, of the beam array.
3. An optical reader/writer according to claim 1, wherein said
means for detecting the beams comprises a beam separator (24, 31,
32, 33) for separating the dedicated focus tracking beam from the
array of beams.
4. An optical reader/writer according to claim 3, wherein said beam
separator (24) comprises a dichroic mirror, arranged to reflect the
array of read-out beams in one direction, and the dedicated focus
tacking beam in a different direction.
5. An optical reader/writer according to claim 3, wherein said beam
separator comprises a diffraction element (32) being adapted to
transmit light having said first wavelength while blocking light
having said second wavelength.
6. An optical reader/writer according to claim 1, wherein said
laser generating means comprises two lasers (21, 22) for generating
two laser beams having said first and second wavelengths
respectively, and a diffraction element (23) arranged in the
optical path of both beams, said diffraction element being adapted
to diffract light having said first wavelength while transmitting
light having said second wavelength.
7. An optical reader/writer according to claim 6, wherein said
diffraction element (23) is a binary grating having a grating depth
essentially satisfying: h(n-1)=1.lamda..sub.1+.phi..sub.step/2.pi.
and h(n-1)=m.lamda..sub.2, where h is the grating depth, n is the
index of refraction of the grating, .lamda..sub.1 and .lamda..sub.1
are the two wavelengths, .phi..sub.step is the desired phase step,
and 1 and m are integers.
8. An optical reader/writer according to claim 1, wherein said
laser generating means comprises a first laser (21) for generating
a first laser beam having said first wavelength, a diffraction
element (12) for diffracting said first laser beam into an array of
laser means, a second laser (22) for generating a second laser beam
having said second wavelength, and means (30) for merging said
array of beams with said second beam.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to an optical reader for a two
dimensional storage disc, comprising means for generating a
plurality of laser beams and projecting said beams onto a rotating
disc, means for detecting the beams after being diffracted by the
disc, and means for determining a focus error signal based on one
of the plurality of beams.
BACKGROUND OF THE INVENTION
[0002] Conventionally, optical storage is performed in one
dimension, i.e. a track of consecutive bits is written onto the
disc (e.g. CD, DVD). Recently, the concept of two dimensional
optical storage has been introduced. The format of a 2D disc is
based on a broad spiral, consisting of a number of parallel bit
rows. Parallel read out is realized using a single laser beam which
passes through a diffraction grating producing an array of spots
scanning the fall width of the broad spiral. Such a system is
disclosed in "Two-Dimensional Optical Storage", by Wim M. J. Coene,
OSA Topical Meetings on Optical Data Storage, May 11-14, 2003,
Technical Digest, pp 90-92.
[0003] For focus tracking of the laser, a focus error signal is
generated using conventional methods (e.g. Foucault, astigmatic,
spot size) based on the central spot of the array. However, the
small separation between spots (in the order of micrometers) causes
the spots to overlap very quickly when out of focus. In the overlap
region the intensity profile is highly distorted because of
interference from adjacent spots, which disturbs the focus signal.
As a result, the capture range, or focus S-curve length, is
significantly reduced. While a conventional one dimensional optical
reader (e.g. a CD ROM drive) has a capture range of around 2-5
micrometers, a two dimensional reader may have a capture range less
than one micrometer. The problem is also present during writing of
a disc.
[0004] With regards to one dimensional optical storage, CD and DVD
systems are typically operated with different types of tracking
methods, i.e. single spot, multiple spot. In order to provide both
types of reading in the same optical reader, such readers are
sometimes provided with two different lasers, having different
wavelengths, and a wavelength dependent optical element in the
laser path. Such a reader is disclosed in EP 1069555. The optical
element is a grating made of a birefringent material, adapted to
act as a grating and thus diffracting light from one of the lasers
(having one of the wavelengths), while leaving light from the
second laser (having another wavelength) unaffected.
SUMMARY OF THE INVENTION
[0005] It is an object of the present invention to overcome this
problem, and to provide a two dimensional optical reader/writer
with improved focus tracking.
[0006] It is a further object to provide a two dimensional optical
reader/writer with improved capture range.
[0007] These and other objects are achieved by an optical
reader/writer of the kind mentioned in the introduction, wherein
said plurality of beams comprises an array of beams having a first
wavelength, and a dedicated focus tracking beam having a second
wavelength, and wherein said focus error signal is based on said
focus tracking beam. According to the invention, beams having one
wavelength is used for the actual accessing of the data on the
disc, while a beam having a second wavelength is used for focus
tracking. The focus tracking can then be based on one single beam,
without interference from adjacent beams.
[0008] The focus tracking beam may coincide with one of the beams,
preferably the central beam, of the beam array. This ensures that
the reflected beam used for focus tracking is reflected in a spot
that is actually used during read-out/writing. The likelihood of
achieving acceptable focus in most of the array beams (i.e. even if
they are mutually unaligned) is increased by using the central
beam.
[0009] The means for detecting the beams preferably comprises a
beam separator for separating the dedicated focus tracking beam
from the array of beams. This provides for separation of the
dedicated focus tracking beam from the read-out/writing beams, and
thus facilitates application of tracking methods, such as the
Foucault method. In the case of reading, the read-out beams must
also be separated in order to enable processing of the high
frequency data, while in the case of writing, it may be enough to
distinguish the focus beam. The beam separator can comprise a
dichroic mirror, arranged to reflect the array of read-out beams in
one direction, and the dedicated focus tracking beam in a different
direction. Such wavelength dependent beam splitters are known in
the art.
[0010] Alternatively, the beam separator can comprise a diffraction
element being adapted to transmit light having said first
wavelength while blocking light having said second wavelength. Such
a wavelength dependent diffraction element can be realized by means
of a binary grating.
[0011] According to one embodiment of the invention, the laser
generating means can comprise two lasers for generating two laser
beams having said first and second wavelengths respectively, and a
diffraction element arranged in the optical path of both beams,
said diffraction element being adapted to diffract light having
said first wavelength while transmitting light having said second
wavelength.
[0012] Such a wavelength dependent diffraction element is known per
se, and is described e.g. in EP 1069555, mentioned above. However,
in the reader according to EP 1069555, light of only one wavelength
at a time is used selectively to read different types of optical
discs (CD and DVD respectively), and the purpose of the diffraction
element is to provide different diffraction depending on the
currently selected wavelength. The purpose of the present invention
is instead, as mentioned above, to provide a dedicated focus
tracking beam by using laser light of two different wavelengths
simultaneously. Letting the two laser beams pass through a
diffraction element as described in EP 1069555 is only one possible
embodiment of the invention.
[0013] The diffraction element can be a binary grating having a
grating depth essentially satisfying:
h(n-1)=1.lamda..sub.1+.phi..sub.step/2.pi. and
h(n-1)=m.lamda..sub.2,
where h is the grating depth, n is the index of refraction of the
grating, .lamda..sub.1 and .lamda..sub.1 are the two wavelengths,
.phi..sub.step is the desired phase step, and 1 and m are integers.
Such a grating is reasonably easy to realize.
[0014] According to a different embodiment, the laser generating
means comprises a first laser for generating a first laser beam
having said first wavelength, a diffraction element for diffracting
said first laser beam into an array of laser beams, a second laser
for generating a second laser beam having said second wavelength,
and means for merging said array of beams with said second
beam.
[0015] This embodiment does not require a wavelength dependent
grating as mentioned above, but instead merges laser beams having
different wavelengths together after one of them has been
diffracted into an array of beams.
[0016] These and other aspects of the invention will be apparent
from and will be elucidated with reference to the embodiments
described hereinafter.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] The present invention will now be described in more detail,
by way of example, with reference to the accompanying drawings,
wherein:
[0018] FIG. 1 shows the layout of two dimensional storage on an
optical disc,
[0019] FIG. 2 shows parallel read-out of the disc in FIG. 1
according to prior art,
[0020] FIG. 3 shows schematically a set-up for an optical reader
according to a first embodiment of the present invention,
[0021] FIG. 4 shows schematically a set-up for an optical reader
according to a second embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0022] The principles of two dimensional storage on an optical disc
1 is illustrated in FIG. 1. The information is stored in a broad
spiral 2, comprising a number of parallel bit-rows 3, here five
rows, and a guard band 4. In the example in FIG. 1, the bit-rows 3
are aligned with each other in the radial direction to form a
hexagonal lattice of bits. This means that each bit 5, 6 is
associated with a physical hexagonal bit-cell 7, 8. Typically, the
bit-cell 7 of a bit with value zero has a uniformly flat area,
while a bit-cell 8 for a bit with value one has a hole 9 centrally
in the hexagonal area. The size of such a hole 9 is preferably
comparable with or smaller than half of the bit-cell area, in order
to eliminate signal folding, i.e. a cluster of zeroes and a cluster
of ones would both result in a perfect mirror.
[0023] FIG. 2 shows how parallel read-out from the disc in FIG. 1
is realized conventionally, using a laser beam 11 which passes
through a diffraction grating 12 which produces an array of beams
13 which are focused onto the disc 1 by a collimator lens 14 and an
objective lens 15, to form an array of spots across the entire
width of the spiral 2. Each beam 13 is reflected and diffracted by
the disc 1, and is then reflected by a beam splitter 16 and
detected by a multi-partitioned photo-detector 17 which generates a
number of high frequency waveforms used as input for 2D signal
processing, performed in a processor 18. The processor 18 also
provides a focus tracking signal 19 to the objective lens 15, by
calculating a focus error signal based on the central spot. Such a
system is described in "Two-Dimensional Optical Storage", by Wim M.
J. Coene, OSA Topical Meetings on Optical Data Storage, May 11-14,
2003, Technical Digest, pp 90-92, herewith incorporated by
reference.
[0024] A first embodiment of the invention is shown in FIG. 3,
where elements corresponding to elements in FIG. 2 are denoted with
identical reference numerals. According to the invention, the
optical reader comprises two lasers 21, 22 generating laser beams
of different wavelengths, e.g. one red and one blue. One of these
laser beams is then diffracted into an array of beams, while the
second is used as a dedicated focus tracking beam. In the
embodiment in FIG. 3, a wavelength dependent diffraction element
23, here a binary grating, is arranged in the optical path of both
lasers. The grating is adapted to act as a diffraction element for
one of the beams (e.g. the blue beam), while being transparent for
the other beam (e.g. the red beam).
[0025] This can be accomplished with a binary grating where the
grating depth of the grating is such that for one wavelength the
required phase depth is achieved and for the other wavelength the
phase depth is a multiple of 2.pi.. In equation form, this
corresponds to:
h(n-1)=1.lamda..sub.1+.phi..sub.step/2.pi. and
h(n-1)=m.lamda..sub.2,
[0026] where h is the grating depth, n is the index of refraction
of the grating, .lamda..sub.1 and .lamda..sub.1 are the two
wavelengths, .phi..sub.step is the desired phase step, and 1 and m
are integers.
[0027] The two initial laser beams can be arranged to coincide,
such that, after the diffraction, the undiffracted focus beam will
coincide with the central beam of the beam array.
[0028] The beam array and the dedicated focus tracking beam are
then focused onto the disc and reflected in a similar way as was
described above with reference to FIG. 2. The reflected beams are
then directed into a beam separator 24, adapted to separate the
reflected beam array, comprising the high frequency read-out data,
from the reflected focus beam. In the embodiment in FIG. 3, this
separation is accomplished by a wavelength dependent beam splitter,
here including a dichroic mirror.
[0029] The high frequent read-out data is directed to an optical
multi-partitioned photo-detector 25 which generates a number of
high frequency waveforms used as input for 2D signal processing in
a processor 26, essentially in the same way as described above with
reference to FIG. 2. The focus beam is instead directed to another
photo-detector 27 and another processor 28, which generates a focus
tracking signal 29. This signal is used to track the optical system
15, as described above.
[0030] A second embodiment of the invention is shown in FIG. 4,
where only one 21 of the two lasers 21, 22 is directed into a
diffraction element 12 to generate the beam array. This diffraction
element does not have to be wavelength sensitive, but can be of
conventional kind, like the one in FIG. 1. The second laser beam is
then merged into the beam array using a transmissive mirror 30
(i.e. an inverted beam splitter).
[0031] Further, in the embodiment in FIG. 4, the reflected beams
are separated using a beam splitter 31 and two wavelength dependent
diffraction elements 32, 33. Each element 32, 33 is adapted to be
transmissive for one of the wavelengths, and blocking for the
other. This could be implemented in a so-called LDGU structure. As
a result, the high frequency data will pass the element 32 and hit
the optical detector 25, while the focus beam will pass the element
33 and hit the optical detector 27. The farther processing
corresponds to the embodiment in FIG. 3.
[0032] The person skilled in the art realizes that the present
invention by no means is limited to the preferred embodiments
described above. On the contrary, many modifications and variations
are possible within the scope of the appended claims. For example,
although the invention has been described with reference to an
optical reader, the invention is equally applicable to an optical
writer, where the same focus tracking is required.
[0033] Any reference sign in a claim should not be construed as
limiting the claim. Use of the verb "to comprise" and its
conjugations does not exclude the presence of elements or steps
other than those stated in a claim. Use of the article "a" or "an"
preceding an element or step does not exclude the presence of a
plurality of such elements or steps.
* * * * *